The invention is directed to improvements in valves and in the behavior of their valve closing members.
In a fuel injection apparatus of this kind, such as that disclosed in Austrian Pat. No. 312 368, the valve comprises a slide valve, and the pump piston serves as the movable valve closing member, which with an oblique control edge on its jacket face causes the outlet of the relief conduit on the circumferential wall of the pump cylinder to communicate with the pump work chamber beyond a pump supply stroke defined by the rotational position of the pump piston. In this manner, the rotational position of the pump piston determines the amount of fuel injected at high pressure per pumping stroke.
This kind of fuel injection apparatus also includes unit fuel injectors, which combine a fuel injection pump and a fuel injection valve with a very short pressure line of stable shape located between the pump work chamber and the injection valve opening. Unit fuel injectors are used to attain accurate injection even with very high-speed internal combustion engines that operate with direct injection into the combustion chambers. In this case, to attain good fuel preparation given the very short period of time that is available for ignition delay and combustion of the fuel that has been introduced, accurate and very fast control of the injection valve closing members must be assured, and the apparatus must also operate at an elevated injection pressure in order to better atomize the introduced fuel.
In the known fuel injection apparatus, there are limits to the closing speed of the valve closing member, because of the dynamic behavior of the fuel and because of the problems in attaining large relief cross sections, for the sake of a rapid decrease in pressure at the end of injection, and in attaining large spring closing forces. The closing behavior, and at the same time the opening characteristic of the valve, must be taken into account.
In the known apparatus, a particular problem is that the desired characteristic curve of the course of injection cannot be attained at the instant of the end of injection; this desired characteristic would assure that substantially all the fuel metered for fuel injection, in particular in the final phase of injection, would come to be injected at the high injection pressure that is required for good atomization or fuel preparation. Rapid relief of pressure, which is intended to end the injection, is associated with high-amplitude pressure wave formation, cavitation damage, and late injections, caused by pressure wave peak pressures, that occur after the desired end of injection. As is well known, the pressure wave peak pressures must be reduced by appropriately designed pressure valves, but these valves exhibit widely varying behavior over the entire operating range of the engine.
It is also known, from German Offenlegungsschrift No. 2 301 419, to have the above-mentioned oblique control edge on the pump piston open up a connecting conduit to the spring chamber of the injection valve closing member, before it effects communication with the relief conduit. However, in order to prevent blockage of the pressure piston in the stroke portion between the opening of the connecting conduit and the opening of the relief conduit, a pressure maintenance valve is also required, by way of which fuel may flow out in an uncontrollable quantity. This arrangement is intended to attain rapid closure of the valve closing member of the injection valve at the end of fuel supply.
Another feature of this embodiment is a housing separation between the injection pump and the injection valve, which communicate with one another via pressure tube lines. At the moment of the intended end of injection, no substantial relief of the pump work chamber takes place yet, because relief is effected only via the compressibility of the spring chamber volume, if need be via the opening characteristic of the pressure maintenance valve. Only the force of the injection valve spring then acts in the closing direction, because the valve closing member is balanced in pressure. Because of the time delay in relief of the pump work chamber via the relief conduit, this embodiment must also be expected to have a relatively slow closure, possibly in increments, of the valve needle.